Cover glass adhering device

ABSTRACT

Graphite flake is annealed at a temperature of at least about 3000° C. prior to intercalation. This annealing process results in enhanced expansion of intercalated graphite flake and provides uniform expansion of intercalated graphite flake derived from variety of sources.

FIELD OF THE INVENTION

[0001] This invention relates to intercalated graphite flake havingincreased exfoliation volume.

BACKGROUND OF THE INVENTION

[0002] Graphite is a crystalline form of carbon comprising atoms bondedin flat layered planes with weaker bonds between the planes. By treatingparticles of graphite, such as natural graphite flake, with anintercalant of, e.g., a solution of sulfuric and nitric acid, thecrystal structure of the graphite reacts to form a compound of graphiteand the intercalant. The treated particles of graphite are hereafterreferred to as intercalated graphite flake. Upon exposure to elevatedtemperatures the particles of intercalated graphite expand in dimensionin an accordion-like fashion in the c-direction, i.e. in the directionperpendicular to the crystalline planes of the graphite.

[0003] Intercalated graphite flake has many useful applications. Acommon application is to exfoliate the intercalated graphite particlesinto vermicular-like structures which are then compressed into sheets offlexible graphite for use in the manufacture of gaskets or as packingmaterial. Intercalated graphite flake is also used in a variety ofproducts which take advantage of the high expansion which such flakesundergo when exposed to high temperature. One such example is for use incombination with polymer foams to form seat cushions and furnitureupholstery in aircraft. Upon exposure to fire, the high temperature willcause the particles of intercalated graphite to exfoliate whichminimizes or prevents the formation of toxic gases from the polymer foamand may, of itself, smother a fire. Since it is important to suppress,i.e., retard, a fire before it has begun to spread, it would be asubstantial advantage for an intercalated graphite flake product toexhibit a very high degree of exfoliation.

[0004] Also, as is appreciated in the art, graphites from various mines,when intercalated and subjected to exfoliation, exhibit greatly varyingdegrees of expansion. For this reason, naturally occurring graphite fromcertain sources which exhibits poor expansion upon exfoliation isexcluded as a source material for many applications.

[0005] It has been discovered in accordance with the present inventionthat when graphite flake is subjected to a preliminary annealing step,which involves heating to a temperature of at least 3000° C., thegraphite flake upon intercalation and subsequent exfoliation exhibitsenhanced expansion, and that graphite flake from sources previouslyconsidered unacceptable by reason of poor expansion characteristics,achieves expansion comparable to that demonstrated by the flake fromsuperior sources. Heretofore, such an extreme annealing temperature hasnot been employed in the art. Borkowski, U.S. Pat. No. 4,102,960,teaches annealing in the presence of boron, but Borkowski's process wasconducted at a lower temperature, preferably 2750° C. and was designedto achieve boron permeation of the graphite flake, which involves aprocess unrelated to applicant's invention.

SUMMARY OF THE INVENTION

[0006] It is an object of the invention to provide graphite flake whichwhen intercalated will exhibit excellent expansion properties.

[0007] It is another object of the invention to provide graphite flakewhich when intercalated exhibits an enhanced and uniform degree ofexpansion.

[0008] It is a further object of the invention to provide a method fortreating graphite flake prior to intercalation, which treatmentfacilitates the subsequent intercalation of flakes which heretofore hadbeen considered too small to provide satisfactory expansion uponsubsequent intercalation and exfoliation.

[0009] It is a further object of the invention to provide a method fortreating graphite flake prior to intercalation, which treatment willresult in enhanced expansion of flake, even when such flake is derivedfrom natural sources which heretofore had been found to produce flakewith an inferior degree of expansion.

[0010] These and other objects are achieved by the invention whichprovides a method forming particles of intercalated graphite flakehaving enhanced exfoliation volumes. The method involves annealinggraphite flake at a temperature in the range of 3000° C. prior tointercalation and exfoliation.

DETAILED DESCRIPTION OF THE INVENTION

[0011] Intercalated graphite flake is conventionally formed by treatingparticles of natural graphite with agents that intercalate into thecrystal structure of the graphite to form a compound of graphite and theintercalant of expansion in the c-direction, i.e. the directionperpendicular to the crystalline planes of the graphite, when heated toa high temperature of above 700° C. and preferably above 1000° C. Theintercalated graphite flake is washed and dried prior to exfoliation.Exfoliated graphite particles are vermiform in appearance and arecommonly referred to as “worms”.

[0012] The conventional method for the intercalation and expansion, orexfoliation, of graphite flake is described by Shane et al. in U.S. Pat.No. 3,404,061, the disclosure of which is incorporated herein byreference. In the typical practice of the Shane et al. method, naturalgraphite flakes are intercalated by dispersing the flakes in a solutioncontaining e.g., a mixture of nitric and sulfuric acid, advantageouslyat a level of about 20 to about 300 parts by weight of intercalantsolution per 100 parts by weight of graphite flakes (pph). Theintercalation solution contains oxidizing and other intercalating agentsknown in the art. Examples include those containing oxidizing agents andoxidizing mixtures, such as solutions containing nitric acid, potassiumchlorate, chromic acid, potassium permanganate, potassium chromate,potassium dichromate, perchloric acid, and the like, or mixtures, suchas for example, concentrated nitric acid and chlorate, chromic acid andphosphoric acid, sulfuric acid and nitric acid, or mixtures of a strongorganic acid, e.g. trifluoroacetic acid, and a strong oxidizing agentsoluble in the organic acid. Alternatively, an electric potential can beused to bring about oxidation of the graphite. Chemical species that canbe introduced into the graphite crystal using electrolytic oxidationinclude sulfuric acid as well as other acids.

[0013] In a preferred embodiment, the intercalating agent is a solutionof a mixture of sulfuric acid, or sulfuric acid and phosphoric acid, andan oxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. The intercalation solution may also contain metal halides suchas ferric chloride, and ferric chloride mixed with sulfuric acid, or ahalide, such as bromine, as a solution of bromine and sulfuric acid orbromine, in an organic solvent.

[0014] The quantity of intercalation solution may range from about 20 toabout 150 pph and more typically about 50 to about 120 pph. After theflakes are intercalated, any excess solution is drained from the flakesand the flakes are water-washed. Alternatively, the quantity of theintercalation solution may be limited to between about 10 and about 50pph, which permits the washing step to be eliminated as taught anddescribed in U.S. Pat. No. 4,895,713, the disclosure of which is alsoherein incorporated by reference.

[0015] The particles of graphite flake treated with intercalationsolution can optionally be contacted, e.g. by blending with a reducingorganic agent selected from alcohols, sugars, aldehydes and esters whichare reactive with the surface film of oxidizing intercalating solutionat temperatures in the range of 25° C. and 125° C. Such a process istaught in U.S. Pat. No. 6,149,972 to Greinke, the disclosure of which isincorporated herein by reference. Suitable specific organic agentsinclude hexadecanol, octadecanol, 1-octanol, 2-octanol, decylalcohol,1,10 decanediol, decylaldehyde, 1-propanol, 1,3 propanediol,ethyleneglycol, polypropylene glycol, dextrose, fructose, lactose,sucrose, potato starch, ethylene glycol monostearate, diethylene glycoldibenzoate, propylene glycol monostearate, glycerol monostearate,dimethyl oxylate, diethyl oxylate, methyl formate, ethyl formate,ascorbic acid and lignin-derived compounds, such as sodium lignosulfate.The amount of organic reducing agent is suitably from about 0.5 to 4%weight of the particles of graphite flake.

[0016] The use of an expansion aid applied prior to, during orimmediately after intercalation can also provide improvements. Amongthese improvements can be reduced exfoliation temperature and increasedexpanded volume (also referred to as “worm volume”). An expansion aid inthis context will advantageously be an organic material sufficientlysoluble in the intercalation solution to achieve an improvement inexpansion. More narrowly, organic materials of this type that containcarbon, hydrogen and oxygen, preferably exclusively, may be employed.Carboxylic acids have been found especially effective. A suitablecarboxylic acid useful as the expansion aid can be selected fromaromatic, aliphatic or cycloaliphatic, straight chain or branched chain,saturated and unsaturated monocarboxylic acids and polycarboxylic acidswhich have at least 1 carbon atom, and preferably up to about 15 carbonatoms, which is soluble in the intercalation solution in amountseffective to provide a measurable improvement of one or more aspects ofexfoliation. Suitable organic solvents can be employed to improvesolubility of an organic expansion aid in the intercalation solution.

[0017] Representative examples of saturated aliphatic carboxylic areacids such as those of the formula H(CH₂)_(n)COOH wherein n is a numberof from 0 to about 5, including formic, acetic, propionic, butyric,pentanoic, hexanoic, and the like. In place of the carboxylic acids, theanhydrides or reactive carboxylic acid derivatives such as alkyl esterscan also be employed. Representative of alkyl esters are methyl formateand ethyl formate. Sulfuric acid, nitric acid and other known aqueousintercalants have the ability of decompose formic acid, ultimately towater and carbon dioxide. Because of this, formic acid and othersensitive expansion aids are advantageously contacted with the graphiteflake in aqueous intercalant. Representative of dicarboxylic acids arealiphatic dicarboxylic acids having 2-12 carbon atoms, in particularoxalic acid, fumaric acid, malonic acid, maleic acid, succinic acid,glutaric acid, adipic acid, 1,5-pentanedicarbosylic acid,1,6-hexanedicarboxylic acid, 1,10-decanedicarboxylic acid,cyclohexane-1,4-dicarboxylic acid and aromatic dicarboxylic acids suchas phthalic acid or terephthalic acid. Representatives of alkyl estersare dimethyl oxylate and diethyl oxylate. Representatives of alkylesters are dimethyl oxylate and diethyl oxylate. Representative ofcycloaliphatic acids is cyclohexane carboxylic acid and of aromaticcarobxylic acids are benzoic acid, naphthoic acid, anthranilic acid,p-aminobenzoic acid, salicylic acid, o-, m- and p-tolyl acids, methoxyand ethoxybenzoic acids, acetoacetamidobenzoic acids and,acetamidobenzoic acids, phenylacetic acid and napthoic acids.Representative of hydroxy aromatic acids are hydrobenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-2-naphthoic acid,6-hydroxy-2-naphthoic acid and 7-hydroxy-2-naphthoic acid. Prominentamong the polycarboxylic acids is citric acid.

[0018] The intercalation solution will be aqueous and will preferablycontain an amount of expansion aid of from about 1 to 10%, the amountbeing effective to enhance exfoliation. If the expansion aid iscontacted with the graphite flake prior to or after immersing in theaqueous intercalation solution, the expansion aid can be admixed withthe graphite by suitable means, such a V-blender, typically in an amountof from about 0.2% to about 10% by weight of the graphite flake.

[0019] After intercalating the graphite flake, and following theblending of the intercalant coated intercalated graphite flake with theorganic reducing agent, the blend may be exposed to temperatures in therange of 25° to 125° C. to promote reaction of the reducing agent andintercalant coating. The heating period is up to about 20 hours, withshorter heating period, e.g., at least about 10 minutes, for highertemperatures in the above-noted range. Times of one half hour or less,e.g., on the order of 10 to 25 minutes, can be employed at the highertemperatures.

[0020] The thus treated particles of graphite are sometimes referred toas “particles of intercalated graphite.” Typically the intercalatedflake is subjected to a water wash to remove surface acid and otherimpurities. For some applications, it may be desirable to then contactthe intercalated graphite flake with a surfactant as is taught in U.S.Pat. No. 5,376,450 to Greinke et al , the disclosure of which isincorporated herein by reference. This surfactant treatment reduces thelevel of surface acid below the level achievable with a simple waterwash, and may result in enhanced expansion.

[0021] Upon exposure to high temperature, e.g., temperatures of at leastabout 160° C. and especially about 700° C. to 1000° C. and higher, thethus prepared particles of intercalated graphite expand as much as about80 to 10000 or more times their original volume in an accordion-likefashion in the c-direction, i.e. in the direction perpendicular to thecrystalline planes of the constituent graphite particles. The expanded,i.e. exfoliated, graphite particles are vermiform in appearance, and aretherefore commonly referred to as worms, and are sometimes referredherein as “particles of expanded graphite.” The worms may be compressedtogether into flexible sheets that, unlike the original graphite flakes,can be formed and cut into various shapes and provided with smalltransverse openings by deforming mechanical impact as hereinafterdescribed.

[0022] The present invention is based on the discovery that the abovedescribed methods for intercalating and exfoliating graphite flake maybeneficially be augmented by a pretreatment of the graphite flake atgraphitization temperatures, i.e. temperatures in the range of about3000° C. and above. This initial heating, or annealing, of the graphiteflake results in significantly increased expansion (i.e., increase inexpansion volume of up to 300% or greater) when the flake issubsequently subjected to intercalation and exfoliation. Indeed,desirably, the increase in expansion is at least about 50%, as comparedto similar processing without the annealing step. The temperaturesemployed for the annealing step should not be significantly below 3000°C., because temperatures even 100° C. lower result in substantiallyreduced expansion.

[0023] As is appreciated in the art, natural graphite from many mineshas heretofore been considered unsatisfactory, or of marginal utility,for many applications, for the graphite flake from those sources did notundergo satisfactory expansion when subjected to conventionalintercalation and exfoliation. When subjected to the above describedannealing, however, graphite flake from sources previously consideredunsatisfactory becomes usable. In fact, when subjected to the annealingprocedure of the instant invention, graphite flake from a full range ofnatural sources achieves a uniform and unexpectedly enhanced degree ofexpansion.

[0024] In addition to providing uniform and enhanced expansion ofgraphite from all sources, annealing permits satisfactory intercalationand exfoliation of smaller particle size natural graphite then hadheretofore had been deemed suitable for intercalation and exfoliationowing to poor expansion. Following the annealing process of the presentinvention, intercalating and exfoliation of small graphite flake can bemade to yield a unique expanded graphite product.

[0025] The annealing of the present invention is performed for a periodof time sufficient to result in a flake having an enhanced degree ofexpansion upon intercalation and subsequent exfoliation. Typically thetime required will be 1 hour or more, preferably 1 to 3 hours and willmost advantageously proceed in an inert environment. For maximumbeneficial results, the annealed graphite flake will also be subjectedto other processes known in the art to enhance the degreeexpansion—namely intercalation in the presence of an organic reducingagent, an intercalation aid such as an organic acid, and a surfactantwash following intercalation. Moreover, for maximum beneficial results,the intercalation step may be repeated.

[0026] The annealing step of the instant invention may be performed inan induction furnace or other such apparatus as is known and appreciatedin the art of graphitization; for the temperatures here employed, whichare in the range of 3000° C., are at the high end of the rangeencountered in graphitization processes.

What is claimed is:
 1. A process for enhancing the expansion ofintercalated graphite flake comprising annealing the graphite flake at atemperature of at least 3000° C. prior to intercalation.
 2. The processclaim of claim 1 wherein the annealing is performed for a period inexcess of 1 hour.
 3. The process of claim 2 wherein the annealing isperformed in an inert environment.
 4. A process for achieving expansionof graphite flake comprising: (1) annealing the flake at a temperatureof at least 3000° C. for at least about 1 hour; and (2) treating theflake with an intercalant solution to provide intercalated graphiteflake.
 5. The process of claim 4 wherein the annealing is performed inan inert environment.
 6. The process of claim 4 wherein the intercalatedflake is blended with an organic reducing agent.
 7. The process of claim4 wherein an expansion aid is included in the intercalant solution.